In this paper, a bandpass frequency selective surface (FSS) based on substrate-integrated waveguide (SIW) technology is presented for airborne radome application. The proposed FSS element consists of a tapered cross-slot on either side of the substrate surrounded by metallic vias. The structure selectively allows the impinging electromagnetic (EM) wave through it in the specified frequency band. The element shows very stable frequency response for oblique incidence and the sharp roll-off performance characteristics at the edges of operating region in the rejection band. The −10 dB relative bandwidth (BW) of the element is 12.8% from 9.5 to 10.8 GHz with a very good insertion loss (IL) of 0.1 dB. Furthermore, the spectacular advantages of the FSS element based on SIW have been used for modeling the airborne radome application. The proposed radome structure is operating at 10 GHz with a relative 10 dB BW of 25.0% (8.20–10.6 GHz) with maximum (IL) better than 0.1 dB in its passband. The behavior of the FSS radome has been analyzed at different conformal sectors and thickness is optimized for optimal performance. The key parameters of radome such as transmission performance, radiation efficiency, and radar cross section (RCS) have been studied. Experimental verifications are carried out to prove the validity of the estimated results. The results show promising performance of the proposed FSS based on SIW technology for airborne radome application.

EM Design and Analysis of Frequency Selective Surface Based on Substrate-Integrated Waveguide Technology for Airborne Radome Application

Utilization of fast surrogate models has become a viable alternative to direct handling of full-wave electromagnetic (EM) simulations in EM-driven design. Their purpose is to alleviate the difficulties related to high computational cost of multiple simulations required by the common numerical procedures such as parametric optimization or uncertainty quantification. Yet, conventional data-driven (or approximation) modeling techniques are severely affected by the curse of dimensionality. This is a serious limitation when it comes to modeling of highly nonlinear antenna characteristics. In practice, general-purpose surrogates can be rendered for the structures described by a few parameters within limited ranges thereof, which is grossly insufficient from the utility point of view. This paper proposes a novel modeling approach involving variable-fidelity EM simulations incorporated into the recently reported nested kriging modeling framework. Combining the information contained in the densely sampled low- and sparsely sampled high-fidelity models is realized using co-kriging. The resulting surrogate exhibits the predictive power comparable to the model constructed using exclusively high-fidelity data while offering significantly reduced setup cost. The advantages over conventional surrogates are pronounced even further. The presented modeling procedure is demonstrated using two antenna examples and further validated through the application case studies.

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Antenna Modeling Using Variable-Fidelity EM Simulations and Constrained Co-Kriging

Parametric optimization is a mandatory step in the design of contemporary antenna structures. Conceptual development can only provide rough initial designs that have to be further tuned, often extensively. Given the topological complexity of modern antennas, the design closure necessarily involves full-wave electromagnetic (EM) simulations and—in many cases—global search procedures. Both factors make antenna optimization a computationally expensive endeavor: population-based metaheuristics, routinely used in this context, entail significant computational overhead. This letter proposes a novel approach that interleaves trust-region gradient search with iterative parameter space exploration by means of local kriging surrogate models. Dictated by efficiency, the latter are rendered in low-dimensional subspaces spanned by the principal components of the antenna response Jacobian matrix, extracted to identify the directions of the maximum (frequency-averaged) response variability. The aforementioned combination of techniques enables quasi-global search at the cost comparable to local optimization. These features are demonstrated using two antenna examples as well as benchmarking against multiple-start local tuning.

Quasi-Global Optimization of Antenna Structures Using Principal Components and Affine Subspace-Spanned Surrogates

Over the recent years, surrogate modeling has been playing an increasing role in the design of antenna structures. The main incentive is to mitigate the issues related to high cost of electromagnetic (EM)-based procedures. Among the various techniques, approximation surrogates are the most popular ones due to their flexibility and easy access. Notwithstanding, data-driven modeling of antenna characteristics is associated with serious practical issues, the primary one being the curse of dimensionality, particularly troublesome due to typically high nonlinearity of antenna responses. This limits applicability of conventional surrogates to simple structures described by a few parameters within narrow ranges thereof, which is grossly insufficient from the point of view of design utility. Many of these issues can be alleviated by the recently proposed constrained modeling techniques that restrict the surrogate domain to regions containing high-quality designs with respect to the relevant performance figures, which are identified using the pre-optimized reference designs at an extra computational effort. This paper proposes a methodology based on gradient-enhanced kriging (GEK). It enables a considerable reduction of the number of reference points required to construct the inverse surrogate (employed in surrogate model definition) by incorporating the sensitivity data into the nested kriging framework. Using two antenna examples, it is demonstrated to yield significant savings in terms of the surrogate model setup cost as compared to both conventional modeling methods and the original nested kriging.

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Cost-Efficient Bi-Layer Modeling of Antenna Input Characteristics Using Gradient Kriging Surrogates

A single-screen double-square-loop (DSL) frequency selective surface (FSS) has been developed to multiplex the three frequency (S, X, and Ku) bands for NASA's CRAF/CASSINI mission. This triband DSL FSS reflects the X-band (7.2 and 8.4 GHz) waves and transmits the S-band (2.3 GHz) and the Ku-band (13.8 GHz) waves. A good agreement between predicted and measured transmission performance is found on a 0.001-in.-thick Kapton with/without a flat Kevlar honeycomb sandwich panel. It is found that the single-screen DSL FSS with/without the Kevlar honeycomb has a resonant frequency of 8.4 GHz for incident angle steered from normal to 45 deg and for both TE and TM polarizations.

Single-screen triband FSS with double-square-loop elements. [for CRAF/Cassini mission

Streamlined nosecone radomes for airborne applications have to cater to high-end electromagnetic (EM) performance requirements of fire control radar antenna system. In this regard, the EM performance analysis of an ogival radome based on novel graded dielectric inhomogeneous wall structure is presented. The radome wall considered here consists of seven dielectric layers cascaded in such a way that the middle layer has maximum dielectric parameters (dielectric constant and electric loss tangent) and on either side, dielectric parameters of the layers decrease in a graded (or stepwise) manner. Further, the outer surface of the radome wall is coated with an antistatic and antierosion radome paint. The EM performance parameters of the radome enclosing an X-band slotted waveguide planar array antenna (center frequency: 10 GHz; bandwidth: 1 GHz) are computed based on 3-D ray tracing in conjunction with aperture integration method. The study shows that the proposed graded dielectric inhomogeneous streamlined radome is an excellent choice for airborne applications as compared to airborne radomes based on conventional constant thickness radome designs and variable thickness radome (VTR) designs. Further, it circumvents the constraints on fabrication that occur in streamlined VTR designs.

Electromagnetic Performance Analysis of Graded Dielectric Inhomogeneous Streamlined Airborne Radome

Radome wall configurations have to be modified in order to attain better performance parameters such as high transmission efficiency, low cross-polarization levels, low boresight error, etc., which is an arduous task. Conventional methods followed to obtain better EM performance parameters over a broadband of frequencies are use of thin parallel metallic wire arrays and wire meshes as detailed before in previous chapters. Conventional monolithic radome panels when loaded periodically with metallic inclusions offered much wider bandwidth as discussed in previous chapters.

Broadbanding Techniques Based on Frequency Selective Surfaces (FSS)

Electromagnetic Performance Analysis of Graded Dielectric Inhomogeneous Radomes

The interaction of EM waves with the radome wall must be considered based on first-order effects (such as amplitude and phase distortions) during the EM design of the radome wall. The estimations of the basic performance parameters primarily depend on the EM material parameters of constituent layers of radome wall (dielectric constant, electric loss tangent, etc.), thickness of constituent layers, dimensions and periodicities of unit cell elements of embedded structures, operating frequency and polarization of the antenna system, and the range of incidence angles by the antenna beam during scanning.

EM Performance Characteristics of Radomes

Few works have been reported regarding the application of metallic strip gratings for radomes (Primich in IRE Trans Antennas Propag AP-5:176–182, 1957).

M. Suprava

Papers

Electromagnetic Performance Analysis of Graded Dielectric Inhomogeneous Streamlined Airborne Radome

Broadbanding Techniques Based on Frequency Selective Surfaces (FSS)

Electromagnetic Performance Analysis of Graded Dielectric Inhomogeneous Radomes

EM Performance Characteristics of Radomes

Broadbanding Techniques Based on Metallic Strip Gratings